1. Field of the Invention
The invention relates to a method for desiliconizing pig iron prior to subsequent refining to steel.
2. Description of the Related Art
Blast furnace pig iron, as a rule, contains approximately 0.4 to 2.8% silicon and more than 4% by weight carbon. In a refining procedure, silicon is oxidized to SiO.sub.2 and carbon to carbon monoxide, wherein also an iron burn-up caused by the supply of oxygen can, of course, not be prevented in the course of the refining process. The slag forming during the refining of steel, as a rule, occurs as a strongly basic slag, since an accordingly high lime batch has to be charged, which is partially neutralized by the SiO.sub.2 formed. The high basicity of such steel slags as well as the high portion of iron and chromium oxides as well as, optionally, heavy metal oxides do not readily allow for the direct use of steel slags for reasons of a potential toxicity. While blast furnace slag exhibits favorable hydraulic properties and a substantially reduced content of iron oxide and hence may more easily be supplied to an exploitation as a basic construction material, the disposal of steel works slags becomes more and more difficult, because such steel works slags in the composition as obtained, i.e., without subsequent metallurgical processing, are not readily usable for structural purposes or the like. The metallurgical processing of steel works slags usually involves a high energy consumption and high investment costs.
From EP 666 930 B1, a process has already become known, in which steel slag has been used for refining pig iron. In that known process, the high content of iron oxide of the molten steel slag is utilized for refining molten pig iron having relatively high carbon and silicon contents. Thereby, iron oxide reacts with carbon or iron carbide to form iron and carbon monoxide, whereas the iron oxide of the slag reacts with the silicon of the pig iron bath to form iron and SiO.sub.2.
These reactions are relatively strongly exothermic, and with a long reaction time as in correspondence with the known proposal it is immediately feasible to recover crude steel, wherein the slag parameters may be optimized with a view to the subsequent use within the scope of hydraulically active binders by admixing additives such as CaCO.sub.3, Al.sub.2 O.sub.3 and/or SiO.sub.2, particularly in separate converters.
When refining pig iron, in particular when refining highly silicon-containing pig iron, a quite considerable amount of SiO.sub.2 is formed, which leads to an increase in the lime batch required for the refining process.